Fix OPT_POST_GRT_WNS and disable for some PDKs

[jfgava]

There is no need to run vt_swap and reroute for PDKs that don't benefit from it.
In this PR I disabled this optimization for Sky130HD/HS, GF180, Nangate45, IHP-SG13G2.

The reroute move does not account for capacitance, which is more relevant to timing closure on these PDKs.
It is possible to reroute a net to a lower-resistance path, but at significantly higher capacitance, which degrades timing.

Currently, we are missing an estimate_parasitics call before running the additional optimizations.
And as we are not changing placement with Reroute and VtSwap, there is no need for a legalization + incremental GRT post-repair_setup.

This prevents issues with another PR with Reroute move enhancements

designs/sky130hd/microwatt/rules-base.json updates:

Metric	Old	New	Type
globalroute__antenna_diodes_count	1747	1692	Tighten
detailedroute__antenna__violating__nets	0	1	Failing
detailedroute__antenna_diodes_count	1297	1295	Tighten
finish__timing__setup__tns	-312.0	-307.0	Tighten
finish__timing__hold__tns	-5.02	-4.65	Tighten

[gemini-code-assist]

Code Review

This pull request refactors the global routing helper script to execute incremental global routing, detailed placement, and parasitics estimation unconditionally, while restricting the post-repair timing optimization to run conditionally based on the OPT_POST_GRT_WNS environment variable. Additionally, it defines OPT_POST_GRT_WNS as 0 across multiple platform configuration files. The reviewer feedback suggests using the conditional assignment operator ?= instead of = for OPT_POST_GRT_WNS in the platform configuration files to allow design-specific overrides.